Many people equip homes and businesses with alarm systems to provide increased security for their homes and businesses. Alarm systems may include control panels that a person may use to control operation of the alarm system and sensors that monitor for security breaches. In response to an alarm system detecting a security breach, the alarm system may generate an audible alert and, if the alarm system is monitored by a monitoring service, the alarm system may send electronic data to the monitoring service to alert the monitoring service of the security breach. In addition, carbon dioxide sensors may be used to sense carbon dioxide levels in an enclosed space.